The present invention relates to a liquid-crystalline medium, to the use thereof for electro-optical purposes, and to displays containing this medium, and to novel compounds.
The main use of liquid crystals is as dielectrics in display devices since the optical properties of such substances can be affected by an applied voltage. Electro-optical devices based on liquid crystals are extremely well known to persons skilled in the art and may be based on various effects. Examples of devices of this type are cells having dynamic scattering, DAP cells (deformation of aligned phases), guest/host cells, TN cells having a twisted nematic structure, STN cells (xe2x80x9csupertwisted nematicxe2x80x9d), SBE cells (xe2x80x9csuperbirefringence effectxe2x80x9d) and OMI cells (xe2x80x9coptical mode interferencexe2x80x9d). The most common display devices are based on the Schadt-Helfrich effect and have a twisted nematic structure.
The liquid-crystal materials must have good chemical and thermal stability and good stability towards electrical fields and electromagnetic radiation. Furthermore, the liquid-crystal materials should have low viscosity and give short addressing times, low threshold voltages and high contrast in the cells. Furthermore, at customary operating temperatures, i.e., in the broadest possible range above and below room temperature, they should have a suitable mesophase, for example, a nematic or cholesteric mesophase for the abovementioned cells. Since liquid crystals are generally used as mixtures of a plurality of components, it is important that the components are readily miscible with one another. Further properties, such as electrical conductivity, dielectric anisotropy and optical anisotropy, must meet various requirements depending on the cell type and the area of application. For example, materials for cells having a twisted nematic structure should have positive dielectric anisotropy and low electrical conductivity.
For example, the media desired for matrix liquid-crystal displays containing integrated nonlinear elements for switching individual pixels (MLC displays) are those having high positive dielectric anisotropy, broad nematic phases, relatively low birefringence, very high specific resistance, good UV and temperature stability of the resistance and low vapor pressure.
Matrix liquid-crystal displays of this type are known. Examples of nonlinear elements which can be used to individually switch the individual pixels are active elements (i.e. transistors). This is then referred to as an xe2x80x9cactive matrixxe2x80x9d, and a differentiation can be made between various types, for example:
1. MOS (metal oxide semiconductor) transistors and
2. thin-film transistors (TFTs).
The use of monocrystalline silicon as a substrate material limits the display size since even the modular assembly of the various part displays results in problems at the joints.
In the case of type 1, as in the case of the more promising type 2, which is preferred, the electro-optical effect used is usually the TN effect. In the case of type 2, a differentiation is made between two technologies: TFTs comprising compound semiconductors, such as, for example, CdSe, or TFTs based on polycrystalline or amorphous silicon. Intensive research efforts are being made worldwide in the latter technology.
The TFT matrix is applied to the inside of one glass plate of the display, while the inside of the other glass plate carries the transparent counterelectrode. Compared with the size of the pixel electrode, the TFT is very small and has virtually no adverse effect on the image. This technology can also be extended to fully color-compatible image displays, in which a mosaic of red, green and blue filters is arranged in such a manner that each filter element is located opposite a switchable image element.
The TFT displays usually operate as TN cells with crossed polarizers in transmission and are illuminated from the back.
The term MLC display here covers any matrix display containing integrated nonlinear elements, i.e., in addition to the active matrix, also displays containing passive elements such as varistors or diodes (MIM=metal-insulator-metal).
MLC displays of this type are particularly suitable for TV applications (for example pocket TV sets) or for high-information displays for computer applications (laptops) and in automobile or aircraft construction. In addition to problems with respect to the angle dependence of the contrast and the response times, problems result in MLC displays due to inadequate specific resistance of the liquid-crystal mixtures [TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORIMACHI, K., TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, September 1984: A 210-288 Matrix LCD Controlled by Double Stage Diode Rings, p. 141 ff, Paris; STROMER, M., Proc. Eurodisplay 84, September 1984: Design of Thin Film Transistors for Matrix Adressing [sic] of Television Liquid Crystal Displays, p. 145 ff, Paris]. As the resistance decreases, the contrast of an MLC display worsens and the problem of xe2x80x9cafter image illuminationxe2x80x9d may occur. Since the specific resistance of liquid crystal mixture generally decreases over the life of an MLC display due to interaction with the internal surfaces of the display, a high (initial) resistance is very important to give acceptable service lives. In particular in the case of low-voltage mixtures, it was hitherto not possible to achieve very high specific resistances. It is furthermore important that the specific resistance increases as little as possible with increasing temperature and after heating and/or exposure to UV radiation.
In particular in the case of high-resolution MLC displays, the use of materials from the prior art can have a considerable adverse effect on the image quality due to the occurrence of reversed tilt domains [E. Takahashi et al., Proc. 16th Japan. Liq. Cryst. Conference (1990), 212-213]. The MLC displays of the prior art do not meet current requirements.
It has hitherto been possible to prepare liquid-crystalline media having birefringence and phase range values necessary for practical use (for example clearing point of xe2x89xa770xc2x0) and having threshold voltages of only about 1.8 volts if values of about 98% for the holding ratio under extreme conditions (for example after UV exposure) are desired.
Thus, there continues to be a great demand for MLC displays having very high specific resistance and at the same time a broad operating temperature range, short response times and low threshold voltage which do not have these disadvantages or only do so to a lesser extent.
For TN (Schadt-Helfrich) cells, media are desired which facilitate the following advantages in the cells:
broadened nematic phase range (in particular down to low temperatures),
switchability at extremely low temperatures (outdoor use, automobiles, avionics),
increased stability to UV radiation (longer life).
The media available from the prior art do not make it possible to achieve these advantages while simultaneously retaining the other parameters.
For supertwisted (STN) cells, media are desired which have a greater multiplexing ability and/or lower threshold voltages and low rotational viscosity and/or low frequency dependence of xcex5 and/or broader nematic phase ranges (in particular at low temperatures). To this end, a further extension of the parameter latitude available (clearing point, smectic-nematic transition or melting point, viscosity, dielectric values, elastic values) is urgently desired.
Fluorophenylcyclohexene derivatives of the formulae 
are disclosed in JP 58/018326, JP 57/154136, JP 58/018326, JP 59/082323A and JP 58/198428A.
DE 41 11 765 describes tetra- and pentafluorophenylcyclohexene derivatives of the formula 
DE 41 13 424 C1 mentions the compound 
as an intermediate.
DE 40 35 509 mentions compounds of the formulae 
Compounds of the formula 
in which R1 is trans-4-alkylcyclohexyl, and R2 is H, CN, halogen or alkyl, are disclosed in JP 05/058928.
JP 04/099739 describes fluorine-containing cyclohexene derivatives of the formula 
The object of the invention is to provide media, in particular for MLC, TN or STN displays of this type, which do not have the abovementioned disadvantages or only do so to a lesser extent, and preferably simultaneously have very high specific resistances and low threshold voltages.
It has now been found that this object can be achieved if media according to the invention are used in displays.
In particular, it has surprisingly been found that the media according to the invention exhibit significantly greater surface tilt angles., and the interfering occurrence of reversed tilt domains in MLC displays is thus substantially suppressed. The image quality of the displays according to the invention is thus significantly improved.
The invention thus relates to a liquid-crystalline medium based on a mixture of polar compounds having positive dielectric anisotropy, characterized in that it contains one or more compounds of the general formula I 
in which
R is H, an unsubstituted alkyl or alkenyl radical having up to 18 carbon atoms, in which one or more non-adjacent CH2 groups may be replaced by a radical selected from the group consisting of xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94 and xe2x80x94Cxe2x89xa1Cxe2x80x94, 
X is CN or Qxe2x80x94Y, where
Y is H, F or Cl and
Q is xe2x80x94CF2xe2x80x94, xe2x80x94CHFxe2x80x94, xe2x80x94OCF2xe2x80x94, xe2x80x94OCHFxe2x80x94, xe2x80x94OCH2CF2xe2x80x94, xe2x80x94CHxe2x95x90CFxe2x80x94, xe2x80x94CFxe2x95x90CHxe2x80x94, xe2x80x94CFxe2x95x90CFxe2x80x94, xe2x80x94Oxe2x80x94CHxe2x95x90CFxe2x80x94 or a single bond,
m is 0 or 1, and
n is 0, 1 or 2.
The fluorophenylcyclohexene derivatives of the formula I have significantly better stability on exposure to UV or heat than the corresponding unfluorinated or monofluorinated compounds. Compared with the corresponding hydrogenated compounds, they have a higher xcex94xcex5, which results in surprisingly low threshold voltages in mixtures containing compounds of the formula I.
The invention also relates to electro-optical displays (in particular STN or MLC displays having two plane-parallel outer plates which, together with a frame, form a cell, integrated nonlinear elements for switching individual pixels on the outer plates, and a nematic liquid-crystal mixture of positive dielectric anisotropy and high specific resistance located in the cell) which contain media of this type, and to the use of these media for electro-optical purposes.
The invention likewise relates to novel compounds.
The liquid-crystal mixtures according to the invention facilitate a significant broadening of the parameter latitude available.
The achievable combinations of clearing point, viscosity at low temperature, thermal and UV stability and dielectric anisotropy or threshold voltage are far superior to the previous materials from the prior art.
The requirement for a high clearing point, a nematic phase at xe2x88x9240xc2x0 C. and a high xcex94xcex5 was previously only achievable to an unsatisfactory extent. Although systems such as, for example, ZLI-3119 have a comparable clearing point and comparatively favorable viscosities, they have, however, a xcex94xcex5 of only +3.
Other mixture systems have comparable viscosities and values of xcex94xcex5, but only have clearing points in the region of 60xc2x0 C.
The liquid-crystal mixtures according to the invention make it possible simultaneously to achieve low viscosities at low temperatures (xe2x89xa6600, preferably xe2x89xa6550 mPaxc2x7s at xe2x88x9230xc2x0 C.; xe2x89xa61,800, preferably xe2x89xa61,700 mPaxc2x7s at xe2x88x9240xc2x0 C.) and dielectric anisotropy values xcex94xcex5 of xe2x89xa73.5, preferably xe2x89xa74.0, clearing points above 65xc2x0, preferably above 70xc2x0, and a high value for the specific resistance, which allows excellent STN and MLC displays to be achieved.
It goes without saying that a suitable choice of components of the mixtures according to the invention also allows higher clearing points (for example, above 90xc2x0) to be achieved at higher threshold voltages or lower clearing points to be achieved at lower threshold voltages while retaining the other advantageous properties. The MLC displays according to the invention preferably operate in the first transmission minimum of Gooch and Tarry [C. H. Gooch and H. A. Tarry, Electron. Lett. 10, 2-4, 1974; C. H. Gooch and H. A. Tarry, Appl. Phys., Vol. 8, 1575-1584, 1975]; in this case, a lower dielectric anisotropy is sufficient in addition to particularly favourable electro-optical properties, such as, for example, low gradient of the characteristic line and low angle dependence of the contrast (German Patent 30 22 818) at the same threshold voltage as in an analogous display at the second minimum. This allows significantly higher specific resistances to be achieved in the first minimum using the mixtures according to the invention than using mixtures containing cyano compounds. A person skilled in art can use simple routine methods to establish the birefringence necessary for a prespecified layer thickness of the MLC display through a suitable choice of the individual components and their proportions by weight.
The viscosity at 20xc2x0 C. is preferably xe2x89xa625 mPaxc2x7s. The nematic phase range is preferably at least 70xc2x0, in particular at least 80xc2x0. This range preferably extends at least from xe2x88x9230xc2x0 to +70xc2x0.
Measurements of the xe2x80x9cvoltage holding ratioxe2x80x9d (HR) [S. Matsumoto et al., Liquid Crystals 5, 1320 (1989); K. Niwa et al., Proc. SID Conference, San Francisco, June 1984, p. 304 (1984); G. Weber et al., Liquid Crystals 5, 1381 (1989)] have shown that mixtures according to the invention containing compounds of the formula I exhibit a considerable decrease in the HR with increasing temperature.
The media according to the invention are distinguished by extremely favorable elastic constants and very favorable viscosity values in addition to an unusually broad nematic phase range, resulting, in particular when used in STN displays, in significant advantages over prior-art media.
The media according to the invention are preferably based on a plurality of (preferably two or more) compounds of the formula I, i.e. the proportion of these compounds is xe2x89xa718%, preferably xe2x89xa725%, particularly preferably xe2x89xa735%.
The threshold voltages V10/0/20 achieved are generally xe2x89xa61.8 volts, preferably xe2x89xa61.6 volts and particularly preferably in the range from 1.4 to 1.6 volts, or lower.
The mixtures according to the invention preferably contain fluorophenylcyclohexene derivatives of the subformulae I1 to I7: 
Of these, the compounds of the formula I2 are particularly preferred.
Particular preference is given to mixtures in which the radical X in the compounds of the formula I has the following meaning: F, Cl, OCF3, OCHF2, CFxe2x95x90CHF, OCH2F, OCF2Cl, OCFCl2, CF3, CF2H, CH2F, CF2Cl, OCH2CF3, OCH2CF2H, CFxe2x95x90CF2, CHxe2x95x90CF2, CHxe2x95x90CHF, xe2x80x94Oxe2x80x94CHxe2x95x90CF2, xe2x80x94Oxe2x80x94CHxe2x95x90CFCl, xe2x80x94OCHxe2x95x90CHF, in particular xe2x80x94CFxe2x95x90CF2, xe2x80x94CHxe2x95x90CF2, CN, F, Cl, OCF3, OCHF2OCH2CF3 or OCFxe2x95x90CF2.
n is 0, 1 or 2, preferably 1. m is preferably 0.
If R is an alkyl radical, this can be straight-chain or branched. It is preferably straight-chain, has 2, 3, 4, 5, 6 or 7 carbon atoms and accordingly is preferably ethyl, propyl, butyl, pentyl, hexyl or heptyl, furthermore methyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl or pentadecyl.
Oxaalkyl is preferably straight-chain 2-oxapropyl (=methoxymethyl), 2-(=ethoxymethyl) or 3-oxabutyl (=2-methoxyethyl), 2-, 3- or 4-oxapentyl, 2-, 3-, 4- or 5-oxahexyl, 2-, 3-, 4-, 5- or 6-oxaheptyl, 2-, 3-, 4-, 5-, 6- or 7-oxaoctyl, 2-, 3-, 5-, 6-, 7- or 8-oxanonyl, or 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-oxadecyl.
If R is an alkenyl radical, this can be straight-chain or branched. It is preferably straight-chain and has 2 to 10 carbon atoms. Accordingly, it is in particular vinyl, prop-1- or prop-2-enyl, but-1-, 2- or but-3-enyl, pent-1-, 2-, 3- or pent-4-enyl, hex-1-, 2-, 3-, 4- or hex-5-enyl, hept-1-, 2-, 3-, 4-, 5- or hept-6-enyl, oct-1-, 2-, 3-, 4-, 5-, 6- or oct-7-enyl, non-1-, 2-, 3-, 4-, 5-, 6-, 7- or non-8-enyl, or dec-1-, 2-, 3-, 4-, 5-, 6-, 7-, 8- or dec-9-enyl.
If R is an alkyl radical in which one CH2 group has been replaced by xe2x80x94Oxe2x80x94 and one has been replaced by xe2x80x94COxe2x80x94, these are preferably adjacent. These thus contain an acyloxy group xe2x80x94COxe2x80x94Oxe2x80x94 or an oxycarbonyl group xe2x80x94Oxe2x80x94COxe2x80x94. These are preferably straight-chain and have 2 to 6 carbon atoms. Accordingly, they are in particular acetoxy, propionyloxy, butyryloxy, pentanoyloxy, hexanoyloxy, acetoxymethyl, propionyloxymethyl, butyryloxymethyl, pentanoyloxymethyl, 2-acetoxyethyl, 2-propionyloxyethyl, 2-butyryloxyethyl, 3-acetoxypropyl, 3-propionyloxypropyl, 4-acetoxybutyl, methoxycarbonyl, ethoxycarbonyl, propoxycarbonyl, butoxycarbonyl, pentoxycarbonyl, methoxycarbonylmethyl, ethoxycarbonylmethyl, propoxycarbonylmethyl, butoxycarbonylmethyl, 2-(methoxycarbonyl)ethyl, 2-(ethoxycarbonyl)ethyl, 2-(propoxycarbonyl)ethyl, 3-(methoxycarbonyl)propyl, 3-(ethoxycarbonyl)propyl or 4-(methoxycarbonyl)butyl.
If R is an alkenyl radical in which one CH2 group has been replaced by CO or COxe2x80x94O or Oxe2x80x94COxe2x80x94, this can be straight-chain or branched. It is preferably straight-chain and has 4 to 13 carbon atoms. Accordingly, it is in particular acryloyloxymethyl, 2-acryloyloxyethyl, 3-acryloyloxypropyl, 4-acryloyloxybutyl, 5-acryloyloxypentyl, 6-acryloyloxyhexyl, 7-acryloyloxyheptyl, 8-acryloyloxyoctyl, 9-acryloyloxynonyl, 10-acryloyloxydecyl, methacryloyloxymethyl, 2-methacryloyloxyethyl, 3-methacryloyloxypropyl, 4-methacryloyloxybutyl, 5-methacryloyloxypentyl, 6-methacryloyloxyhexyl, 7-methacryloyloxyheptyl, 8-methacryloyloxyoctyl or 9-methacryloyloxynonyl.
Compounds of the formula I which contain wing groups R which are suitable for polymerization reactions are suitable for the formation of liquid-crystalline polymers.
Compounds of the formula I containing branched wing groups R may occasionally be of importance due to better solubility in the customary liquid-crystalline base materials, but in particular as chiral dopes if they are optically active. Smectic compounds of this type are suitable as components of ferroelectric materials.
Branched groups of this type generally contain not more than one chain branch. Preferred branched radicals R are isopropyl, 2-butyl (=1-methylpropyl), isobutyl (=2-methylpropyl), 2-methylbutyl, isopentyl (=3-methylbutyl), 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl, 2-propylpentyl, isopropoxy, 2-methylpropoxy, 2-methylbutoxy, 3-methylbutoxy, 2-methylpentoxy, 3-methylpentoxy, 2-ethylhexoxy, 1-methylhexoxy, 1-methylheptoxy, 2-oxa-3-methylbutyl, 3-oxa-4-methylpentyl, 4-methylhexyl, 2-nonyl, 2-decyl, 2-dodecyl, 6-methyloctoxy, 6-methyloctanoyloxy, 5-methylheptyloxycarbonyl, 2-methylbutyryloxy, 3-methylvaleryloxy, 4-methylhexanoyloxy, 2-chloropropionyloxy, 2-chloro-3-methylbutyryloxy, 2-chloro-4-methylvaleryloxy, 2-chloro-3-methylvaleryloxy, 2-methyl-3-oxapentyl and 2-methyl-3-oxahexyl.
Compounds of the formula I having SA phases are suitable, for example, for thermally addressed displays.
If R is an alkyl radical in which two or more CH2 groups have been replaced by xe2x80x94Oxe2x80x94 and/or xe2x80x94COxe2x80x94Oxe2x80x94, this may be straight-chain or branched. It is preferably branched and has 3 to 12 carbon atoms. Accordingly, it is in particular biscarboxymethyl, 2,2-biscarboxyethyl, 3,3-biscarboxypropyl, 4,4-biscarboxybutyl, 5,5-biscarboxypentyl, 6,6-biscarboxyhexyl, 7,7-biscarboxyheptyl, 8,8-biscarboxyoctyl, 9,9-biscarboxynonyl, 10,10-biscarboxydecyl, bis(methoxycarbonyl)methyl, 2,2-bis(methoxycarbonyl)ethyl, 3,3-bis(methoxycarbonyl)propyl, 4,4-bis(methoxycarbonyl)butyl, 5,5-bis(methoxycarbonyl)pentyl, 6,6-bis(methoxycarbonyl)hexyl, 7,7-bis(methoxycarbonyl)heptyl, 8,8-bis(methoxycarbonyl)octyl, bis(ethoxycarbonyl)methyl, 2,2-bis(ethoxycarbonyl)ethyl, 3,3-bis(ethoxycarbonyl)propyl, 4,4-bis(ethoxycarbonyl)butyl or 5,5-bis(ethoxycarbonyl)hexyl.
Compounds of the formula I which contain wing groups R which are suitable for polycondensations are suitable for the preparation of liquid-crystalline polycondensates.
The formula I covers both the racemates of these compounds and the optical antipodes, and mixtures thereof.
Of these compounds of the formula I and of the subformulae, preference is given to those in which at least one of the radicals present therein has one of the preferred meanings indicated.
In the compounds of the formula I, preferred stereoisomers are those in which the cyclohexane ring is trans-1,4-disubstituted.
The invention furthermore relates to the cyclohexene derivatives of the formulae I3, I6 and I7.
The invention furthermore relates to fluorophenylcyclohexene derivatives of the formulae 
in which
R and m are as defined in claim 1,
L is H or F and
X is Qxe2x80x94Y, where
Y is H, F or Cl and
Q is xe2x80x94CHFxe2x80x94, xe2x80x94OCF2xe2x80x94, xe2x80x94OCHFxe2x80x94, xe2x80x94OCH2CF2xe2x80x94, xe2x80x94CHxe2x95x90CFxe2x80x94, xe2x80x94CFxe2x95x90CFxe2x80x94, xe2x80x94CFxe2x95x90CHxe2x80x94, xe2x80x94Oxe2x80x94CFxe2x95x90CFxe2x80x94 or xe2x80x94OCHxe2x95x90CFxe2x80x94.
Particular preference is given to compounds in which m is 1.
The compounds of the formula I are prepared by methods known per se, as described in the literature (for example in the standard works, such as Houben-Weyl, Methoden der Organischen Chemie, Georg-Thieme-Verlag, Stuttgart), to be precise under reaction conditions which are known and are suitable for said reactions. Use may also be made here of variants which are known per se, but are not mentioned here in greater detail.
The fluorophenylcyclohexene derivatives of the formula I can be prepared, for example, as follows: 
Preferred embodiments of the liquid-crystalline media according to the invention are given below:
The medium additionally contains one or more compounds selected from the group consisting of the general formulae II, III and IV: 
in which the individual radicals have the following meanings:
R0: alkyl, oxaalkyl, fluoroalkyl or alkenyl, in each case having up to 7 carbon atoms,
X0: F, Cl, CF3, OCF3, OCHF2 or CN,
Y2 and Y2 each H or F,
r: 0 or 1.
The medium additionally contains one or more compounds selected from the group consisting of the general formulae V to VIII: 
in which
R0: alkyl, oxaalkyl, fluoroalkyl or alkenyl, in each case having up to 7 carbon atoms,
X0: F, Cl, CF3, OCF3, OCHF2 or CN,
Y2 and Y2: each, independently of one another, H or F, and r: 0 or 1.
The medium additionally contains one or more compounds selected from the group consisting of the general formulae IX to XII: 
in which R0, X0, Y1 and Y2 each, independently of one another, have one of the meanings given for V-VIII above.
The proportion of compounds of the formulae I to IV together in the total mixture is at least 50% by weight.
The proportion of compounds of the formula I in the total mixture is from 10 to 50% by weight.
The proportion of compounds of the formulae II to IV in the total mixture is from 30 to 70% by weight. 
xe2x80x83is preferably 
The medium contains compounds of the formulae II and III or IV.
R0 is straight-chain alkyl or alkenyl having 2 to 7 carbon atoms.
The medium essentially comprises compounds of the formulae I to IV.
The medium contains further compounds, preferably selected from the following group consisting of the general formulae XIII to XV: 
xe2x80x83in which R0 and X0 are as defined as for V-VIII, above, the 1,4-phenylene rings may be substituted by CN, chlorine or fluorine.
The weight ratio I: (II+III+IV) is preferably from 1:10 to 1:1.
The medium essentially comprises compounds selected from the group consisting of the general formulae I to XV.
It has been found that even a relatively small proportion of compounds of the formula I mixed with conventional liquid-crystal materials, but in particular with one or more compounds of the formula II, III and/or IV, results in a significant increase in the pitch angle and in low values for the birefringence, and at the same time broad nematic phases with low smectic-nematic transition temperatures are observed. The compounds of the formulae I to IV are colorless stable and readily miscible with one another and with other liquid-crystal materials.
The term xe2x80x9calkylxe2x80x9d includes straight-chain and branched alkyl groups having 1-7 carbon atoms, in particular the straight-chain groups methyl, ethyl, propyl, butyl, pentyl, hexyl and heptyl. Group [sic] having 2-5 carbon atoms are generally preferred.
The term xe2x80x9calkenylxe2x80x9d includes straight-chain and branched alkenyl groups having 2-7 carbon atoms, in particular the straight-chain groups. Particularly [sic] alkenyl groups are C2-C7-1E-alkenyl, C4-C7-3E-alkenyl, C5-C7-4-alkenyl, C6-C7-5-alkenyl and C7-6-alkenyl, in particular C2-C7-1E-alkenyl, C4-C7-3E-alkenyl and C5-C7-4-alkenyl. Examples of preferred alkenyl groups are vinyl, 1E-propenyl, 1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl, 3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl, 4E-hexenyl, 4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groups having up to 5 carbon atoms are generally preferred.
The term xe2x80x9cfluoroalkylxe2x80x9d preferably includes straight-chain groups containing terminal fluorine, i.e. fluoromethyl, 2-fluoroethyl, 3-fluoropropyl, 4-fluorobutyl, 5-fluoropentyl, 6-fluorohexyl and 7-fluoroheptyl. However, other positions of the fluorine are not excluded.
The term xe2x80x9coxaalkylxe2x80x9d preferably includes straight-chain radicals of the formula CnH2n+1xe2x80x94Oxe2x80x94(CH2)m in which n and m are each, independently of one another, 1 to 6. n is preferably 1 and m is preferably 1 to 6.
Through a suitable choice of the meanings of R0 and X0, the addressing times, the threshold voltage, the gradient of the transmission characteristic lines, etc., can be modified as desired. For example, 1E-alkenyl radicals, 3E-alkenyl radicals, 2E-alkenyloxy radicals and the like generally give shorter addressing times, improved nematic tendencies and a higher ratio between the elastic constants k33 (bend) and k11 (splay) compared with alkyl and alkoxy radicals. 4-Alkenyl radicals, 3-alkenyl radicals and the like generally give lower threshold voltages and lower values of k33/k11 compared with alkyl and alkoxy radicals.
A xe2x80x94CH2CH2xe2x80x94 group in Z1 or Z2 generally results in higher values of k33/k11 compared with a single covalent bond. Higher values of k33/k11 facilitate, for example, flatter transmission characteristic lines in TN cells with a 90xc2x0 twist (for achieving gray tones) and steeper transmission characteristic lines in STN, SBE and OMI cells (greater multiplexing ability), and vice versa.
The optimum mixing ratio of the compounds of the formulae I and II+III+IV depends substantially on the desired properties, on the choice of the components of the formulae I, II, III and/or IV and on the choice of any other components which may be present. Suitable mixing ratios within the abovementioned range can easily be determined from case to case.
The total amount of compounds of the formulae I to XV in the mixtures according to the invention is not crucial. The mixtures may therefore contain one or more further components in order to optimize various properties. However, the observed effect on the addressing times and the threshold voltage is generally greater the higher the total concentration of compounds of the formulae I to XV.
In a particularly preferred embodiment, the media according to the invention contain compounds of the formula II, III, V and/or VII (preferably II and/or III) in which X0 is CF3, OCF3 or OCHF2. A favorable synergistic effect with the compounds of the formula I results in particularly advantageous properties.
For STN applications, the media preferably contain compounds selected from the group consisting of the formulae V to VIII in which X0 is preferably OCHF2 or CN.
The media according to the invention may furthermore contain a component A comprising one or more compounds of the general formula Ixe2x80x2 having a dielectric anisotropy of from xe2x88x921.5 to +1.5
in which
R1 and R2 are each, independently of one another, n-alkyl, n-alkoxy, xcfx89-fluoroalkyl or n-alkenyl having up to 9 carbon atoms, 
and 
are each, independently of one another,
1,4-phenylene, 2- or 3-fluoro-1,4-phenylene, trans-1,4-cyclohexylene or 1,4-cyclohexenylene,
Z1 and Z2 are each, independently of one another, xe2x80x94CH2CH2xe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, or a single bond,
and
n is 0, 1 or 2.
Component A preferably contains one or more compounds selected from the group consisting of II1 to II7: 
in which R1 and R2 are as defined under the formula Ixe2x80x2.
Component A preferably additionally contains one or more compounds selected from the group consisting of II8 to II20: 
in which R1 and R2 are as defined under the formula Ixe2x80x2, and the 1,4-phenylene groups in II8 to II17 may each, independently of one another, also be monosubstituted or polysubstituted by fluorine.
Furthermore, component A preferably additionally contains one or more compounds selected from the group consisting of II21 to II25: 
in which R1 and R2 are as defined under the formula Ixe2x80x2 and the 1,4-phenylene groups in II21 to II25 may each, independently of one another, also be monosubstituted or polysubstituted by fluorine.
Finally, preferred mixtures of this type are those in which component A contains one or more compounds selected from the group consisting of II26 and II27: 
in which CoH2o+1 is a straight-chain alkyl group having up to 7 carbon atoms.
In some cases, the addition of compounds of the formula 
in which
R1 and R2 are as defined under the formula Ixe2x80x2
and
Z0 is a single bond, xe2x80x94CH2CH2xe2x80x94, 
proves advantageous for suppressing smectic phases, although this may reduce the specific resistance. In order to achieve parameter combinations which are ideal for the application, a person skilled in the art can easily determine whether and, if yes, in what amount these compounds may be added. Normally, less than 15%, in particular 5-10%, are used.
Preference is given to liquid-crystal mixtures which contain one or more compounds selected from the group consisting of Ia to Id: 
in which R2 is n-alkyl having up to 5 carbon atoms.
The type and amount of the polar compounds having positive dielectric anisotropy are not crucial per se. A person skilled in the art can use simple routine experiments to select suitable materials from a wide range of known and, in many cases, also commercially available components and base mixtures. The media according to the invention preferably contain one or more compounds of the formula Ixe2x80x3
in which Z1xe2x80x2, Z2xe2x80x2 and n are as defined under the formula Ixe2x80x2, 
are each, independently of one another, 1,4-phenylene, trans-1,4-cyclohexylene or 3-fluoro-1,4-phenylene, or one of the radicals 
is alternatively trans-1,3-dioxane-2,5-diyl, pyrimidine-2,5-diyl, pyridine-2,5-diyl or 1,4-cyclohexenylene, R0 is n-alkyl, n-alkenyl, n-alkoxy or n-oxaalkyl, in each case having up to 9 carbon atoms, Y1 and Y2 are each, independently of one another, H or F and Xxe2x80x2 is CN, halogen, CF3, OCF3 or OCHF2.
In a preferred embodiment, the media according to the invention for STN or TN applications are based on compounds of the formula Ixe2x80x3 in which Xxe2x80x2 is CN. It goes without saying that smaller or larger proportions of other compounds of the formula Ixe2x80x3 (Xxe2x80x2xe2x89xa0CN) are also possible. For MLC applications, the media according to the invention preferably contain only up to about 10% of nitriles of the formula Ixe2x80x3 (but preferably no nitriles of the formula Ixe2x80x3, but instead compounds of the formula Ixe2x80x2 where Xxe2x80x2 is halogen, CF3, OCF3 or OCHF2). These media are preferably based on the compounds of the formulae II to XV.
The liquid-crystalline media according to the invention preferably contain 2 to 40, in particular 4 to 30, components as further constituents besides one or more compounds according to the invention. These media very particularly preferably contain 7 to 25 components besides one or more compounds according to the invention. These further constituents are preferably selected from nematic or nematogenic (monotropic or isotropic) substances, in particular substances from the classes of the azoxybenzenes, benzylideneanilines, biphenyls, terphenyls, phenyl or cyclohexyl benzoates, phenyl or cyclohexyl esters of cyclohexanecarboxylic acid, phenyl or cyclohexyl esters of cyclohexylbenzoic acid, phenyl or cyclohexyl esters of cyclohexylcyclohexanecarboxylic acid, cyclohexylphenyl esters of benzoic acid, of cyclohexanecarboxylic acid and of cyclohexylcyclohexanecarboxylic acid, phenylcyclohexanes, cyclohexylbiphenyls, phenylcyclohexylcyclohexanes, cyclohexylcyclohexanes, cyclohexylcyclohexylcyclohexenes, 1,4-biscyclohexylbenzenes, 4,4xe2x80x2-biscyclohexylbiphenyls, phenyl- or cyclohexylpyrimidines, phenyl- or cyclohexylpyridines, phenyl- or cyclohexyldioxanes, phenyl- or cyclohexyl-1,3-dithianes, 1,2-diphenylethanes, 1,2-dicyclohexylethanes, 1-phenyl-2-cyclohexylethanes, 1-cyclohexyl-2-(4-phenylcyclohexyl)ethanes, 1-cyclohexyl-2-biphenylylethanes, 1-phenyl-2-cyclohexylphenylethanes optionally halogenated stilbenes, benzyl phenyl ethers, tolans and substituted cinnamic acids. The 1,4-phenylene groups in these compounds may also be fluorinated.
The most important compounds suitable as further constituents of media according to the invention can be characterized by the formulae 1, 2, 3, 4 and 5:
In the formulae 1, 2, 3, 4 and 5, L and E, which may be identical or different, are in each case, independently of one another, a bivalent radical from the group formed by -Phe-, -Cyc-, -Phe-Phe-, -Phe-Cyc-, -Cyc-Cyc-, -Pyr-, -Dio-, -G-Phe- and -G-Cyc- and their mirror images, where Phe is unsubstituted or fluorine-substituted 1,4-phenylene, Cyc is trans-1,4-cyclohexylene or 1,4-cyclohexylene, Pyr is pyrimidine-2,5-diyl or pyridine-2,5-diyl, Dio is 1,3-dioxane-2,5-diyl and G is 2-(trans-1,4-cyclohexyl)ethyl, pyrimidine-2,5-diyl, pyridine-2,5-diyl or 1,3-dioxane-2,5-diyl.
One of the radicals L and E is preferably Cyc, Phe or Pyr. E is preferably Cyc, Phe or Phe-Cyc. The media according to the invention preferably contain one or more components selected from the compounds of the formulae 1, 2, 3, 4 and 5 in which L and E are selected from the group comprising Cyc, Phe and Pyr and simultaneously one or more components selected from the compounds of the formulae 1, 2, 3, 4 and 5 in which one of the radicals L and E is selected from the group comprising Cyc, Phe and Pyr and the other radical is selected from the group comprising -Phe-Phe-, -Phe-Cyc-, -Cyc-Cyc-, -G-Phe- and -G-Cyc-, and optionally one or more components selected from the compounds of the formulae 1, 2, 3, 4 and 5 in which the radicals L and E are selected from the group comprising -Phe-Cyc-, -Cyc-Cyc-, -G-Phe- and -G-Cyc-.
In a smaller sub-group of the compounds of the formulae 1, 2, 3, 4 and 5, Rxe2x80x2 and Rxe2x80x3 are in each case, independently of one another, alkyl, alkenyl, alkoxy, alkoxyalkyl, alkenyloxy or alkanoyloxy having up to 8 carbon atoms. This smaller sub-group is called group A below, and the compounds are labelled with the sub-formulae 1a, 2a, 3a, 4a and 5a. In most of these compounds, Rxe2x80x2 and Rxe2x80x3 are different from one another, one of these radicals usually being alkyl, alkenyl, alkoxy or alkoxyalkyl.
In another smaller sub-group of the compounds of the formulae 1, 2, 3, 4 and 5 which is known as group B, Rxe2x80x3 is xe2x80x94F, xe2x80x94Cl, xe2x80x94NCS or xe2x80x94(O)iCH3xe2x88x92(k+1)FkCll, where i is 0 or 1, and k+l is 1, 2 or 3; the compounds in which Rxe2x80x3 has this meaning are labelled with the sub-formulae 1b, 2b, 3b, 4b and 5b. Particular preference is given to those compounds of the sub-formulae 1b, 2b, 3b, 4b and 5b in which Rxe2x80x3 is xe2x80x94F, xe2x80x94Cl, xe2x80x94NCS, xe2x80x94CF3, xe2x80x94OCHF2 or xe2x80x94OCF3.
In the compounds of the sub-formulae 1b, 2b, 3b, 4b and 5b, Rxe2x80x2 is as defined for the compounds of the sub-formulae 1a-5a and is preferably alkyl, alkenyl, alkoxy or alkoxyalkyl.
In a further smaller sub-group of the compounds of the formulae 1, 2, 3, 4 and 5, Rxe2x80x3 is xe2x80x94CN; this sub-group is known as group C below, and the compounds of this sub-group are correspondingly described by sub-formulae 1c, 2c, 3c, 4c and 5c. In the compounds of the sub-formulae 1c, 2c, 3c, 4c and 5c, Rxe2x80x2 is as defined for the compounds of the sub-formulae 1a-5a and is preferably alkyl, alkoxy or alkenyl.
In addition to the preferred compounds of groups A, B and C, other compounds of the formulae 1, 2, 3, 4 and 5 having other variants of the proposed substituents are also customary. All these substances can be obtained by methods which are known from the literature or analogously thereto.
Besides compounds of the formula I according to the invention, the media according to the invention preferably contain one or more compounds selected from group A and/or group B and/or group C. The proportions by weight of the compounds from these groups in the media according to the invention are preferably
Group A: 0 to 90%, preferably 20 to 90%, in particular 30 to 90%
Group B: 0 to 80%, preferably 10 to 80%, in particular 10 to 65%
Group C: 0 to 80%, preferably 5 to 80%, in particular 5 to 50%,
the sum of the proportions by weight of the group A and/or B and/or C compounds present in the particular media according to the invention preferably being 5% to 90% and in particular 10% to 90%.
The media according to the invention preferably contain 1 to 40%, particularly preferably 5 to 30%, of compounds according to the invention. Further preferred media are those which contain more than 40%, in particular 45 to 90%, of compounds according to the invention. The media preferably contain three, four or five compounds according to the invention.
The construction of the STN and MLC displays according to the invention from polarizers, electrode base plates and electrodes with surface treatment corresponds to the construction which is conventional for displays of this type. The term conventional construction here is widely drawn and also covers all derivatives and modifications of the MLC display, in particular also matrix display elements based on poly-Si TFTs or MIMs.
An essential difference between the displays according to the invention and those customary hitherto based on the twisted nematic cell is, however, the choice of liquid-crystal parameters in the liquid-crystal layer.
The liquid-crystal mixtures which can be used according to the invention are prepared in a manner which is conventional per se. In general, the desired amount of the components used in a lesser amount is dissolved in the components making up the principal constituent, expediently at elevated temperature. It is also possible to mix solutions of the components in an organic solvent, for example acetone, chloroform or methanol, and, after thorough mixing, to remove the solvent again, for example by distillation.
The dielectrics may also contain other additives known to those skilled in the art and described in the literature. For example, 0-15% of pleochroic dyes or chiral dopes can be added.
In the following discussion, C denotes a crystalline phase, S a smectic phase, SB a smectic B phase, N a nematic phase and I the isotropic phase.
V10 denotes the voltage for 10% transmission (viewing direction perpendicular to the plate surface). ton denotes the switch-on time and toff the switch-off time at an operating voltage corresponding to 2.5 times the value of V10. An denotes the optical anisotropy and no the refractive index. xcex94xcex5 denotes the dielectric anisotropy (xcex94xcex5=xcex5∥xc2x7xcex5xe2x8axa5, where xcex5∥ is the dielectric constant parallel to the longitudinal molecular axes and xcex5xe2x8axa5 is the dielectric constant perpendicular thereto. The electro-optical data were measured in a TN cell in the 1st minimum (i.e. at a d xcex94n value of 0.5 mm) at 20xc2x0 C., unless expressly stated otherwise. The optical data were measured at 20xc2x0 C., unless expressly stated otherwise.
In the present application and in the examples below, the structures of the liquid-crystal compounds are indicated by acronyms, with the transformation into chemical formulae taking place in accordance with Tables A and B below. All radicals CnH2n+1 are straight-chain alkyl radicals containing n carbon atoms. The coding in Table B is self-evident. In Table A, only the acronym for the base structure is given. In individual cases, the acronym for the base structure is followed, separated by a hyphen, by a code for the substituents R1, R2, L1 and L2:
xe2x80x9cConventional work-upxe2x80x9d means that water is added, the mixture is extracted with dichloromethane, the phases are separated, the organic phase is dried and evaporated, and the product is purified by crystallization and/or chromatography.
In addition, the abbreviations have the following meanings:
K: crystalline solid state, S: smectic phase (the index denotes the phase type), N: nematic state, Ch: cholesteric phase, I: isotropic phase. The number between two symbols indicates the transition temperature in degrees celsius.